Overmap type can be changed without applying generated terrain

Small change to the map editor.

I'm creating a massive custom map, placing tiles with the map editor. When I want to create a huge building, it's a pain to generate the tile I want it to show up as on the overmap, delete everything that was generated, then place down the tiles I want (either that or edit the map file, which is no fun).

So I added a quick option to change the overmap type without actually applying the generated terrain. Great for making large custom maps!